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Abstract

We report diffuse reflection imaging in air of concealed powdered samples using a terahertz quantum cascade laser. The sensitivity of the detection scheme to sub-surface absorption within samples is confirmed using fully-characterized powdered admixtures of polystyrene and polymethyl methacrylate (PMMA). Measurements of the backscattering intensity from these samples are then used in conjunction with Kubelka-Munk scattering theory, as well as several models based on the quasi-crystalline approximation, to extract the absorption coefficient of PMMA. Our research demonstrates the feasibility of high-resolution frequency-domain terahertz imaging for the detection and identification of concealed powders in a reflection geometry.

Measurement of the modulation transfer function (MTF) of the imaging setup using vertically (blue) and horizontally (red) oriented targets. The lines are intended only to guide the eye. The 20% modulation threshold indicates spatial resolutions equal to 350 µm and 305 µm respectively.

(a) Photograph of the sample holder (lid removed) loaded with powder admixtures. The % mass of PMMA is shown for each admixture. (b) Specular THz image of the sample shown in (a). (c) Diffuse THz image of the sample shown in (a). The non-uniform intensity distributions observed here are attributed to the presence of diffuse surface reflections and scattering from micro-cavities formed within the samples. Such effects are averaged-out with repeated measurements. All three images are displayed on the same scale. The THz images have a pixel size of 250 µm.

Measurement of the sample reflectivity R∞ (black, right axis) as a function of volume fraction of PMMA. The corresponding values of the Kubelka-Munk remission function F(R∞) are also shown (red, left axis). The lines represent fits of the data to Eq. (2), in which the estimated value of γ=1.6 is used, and S=5±1 cm-1 and α1=42±11 cm-1are treated as free parameters.